kerberos auth

Hello!

I didn't find kerberos authentication in Ruby ​​version. Is it implemented?

How can I do this, for example, 
```
vault login -method=kerberos \
            -address=${ADDRESS} \
            username=${USER} \
            realm=${REALM} \
            service=${SERVICE} \
            keytab_path=${KEYTAB} \
            krb5conf_path=${KRB5_CONF}
```
in Ruby?
